Chromatic aberration control and countermeasures for continuous dyeing of reactive dyes

Reactive dyes are reactive dyes, and the dyeing properties of the dyes are not uniform. It is difficult to control the color shade in the continuous pad dyeing production, and it is easy to produce color difference before and after or left and right. Continuous pad dyeing commonly used vinyl sulfone reactive dyes or a chlorine s-triazine and vinyl sulfone double reactive dyes. There are many factors that affect the dyeing of cotton fibers, and there are many reasons for the color difference.

1 poor early staining

Poor initial staining, color difference between initial driving and normal production. Due to the different affinities of different dyes, the dyestuffs will have a competitive dyeing phenomenon when the dyestuffs are dyed in the fabric. The higher the dye concentration in the dyeing solution taken by the fabric, the higher the affinity of the dyes, and the more serious this phenomenon will be. As a result, the concentration of the dyeing solution in the rolling groove gradually decreases, and even the concentration ratio between the dyes (i.e., color difference) is generated until the dynamic balance (stability of the colored light) is achieved under the stable dyeing solution.

2. Color difference in normal continuous production

Mainly due to changes in the padding process parameters or changes in the state of dyeing.

3. Chromatic aberration caused by pretreatment or finishing of dyeing

The color difference caused by the pretreatment of the dyed blank, that is, the color difference caused by the inconsistent effect of desizing, scouring, bleaching, and mercerizing of the dyed blank, and the color difference caused by the post-finishing, ie, the post-finishing temperature is too high or the color change due to the chemical aid is poor.

II. Analysis and Control of Color Differences

1 Analysis and control of poor initial staining

The difference in the initial dyeing is mainly related to the dye properties. Since the dyestuff has a higher affinity for cotton fibers than water molecules, the phenomenon of deep dyeing will occur. It is generally controlled by adding water to the trough or adding dye to the trough. Pad dyeing uses reactive dyes. In consideration of factors such as shade, color fastness and cost, it is also necessary to consider dye affinity, reactivity, alkali resistance and other issues, because these factors will affect the stability of colored light.

1.1 Dye affinity

To select the dye with a lower affinity (Rf values ​​around 0.8), the affinity between the dyes should be close. Affinity is too low, easy to swim when drying (such as corduroy easy to produce pit shallow), cloth color is not full, or even affect the color fastness; affinity is too high, the color is dark, easy to change by the level of the rolling tank The influence of other factors, although the depth of color can be controlled by adding water to the initial bath of the groove, it is not conducive to the control of color shade during continuous dyeing, and the high affinity of the dye is also not conducive to fabric dyeing; the affinity of the selected dye is too different Large, it is even more inconvenient for the control of shades of light, and it tends to produce differences in hue before and after or in the left, center, and right.

Affinity is expressed as the speed at which the dye is transferred from the dye liquor to cotton fibers. The higher the affinity, the faster the dye is transferred from the dye liquor to the cotton fiber and the higher the initial dyeing rate. The affinity of a single dye is usually expressed as a shift value (Rf value). Rf values ​​of different dyes are often different, and high Rf values ​​indicate low affinity. Generally used filter paper dye uptake method (dye concentration 10g / L, room temperature, test dye within 5 min on the medium filter paper dye and water height ratio). It is particularly worth noting that the dyes have the same shift value and the affinity is not necessarily the same. Since commercial dyes contain surfactants such as defoamers, dust-reducing agents, and solubilizers, as well as inorganic salt electrolytes such as Yuan Mingfen, these additives can seriously affect the Rf value of dyes, and the percentage of additives contained in dyes of different specifications is often not same. Therefore, the measured individual dye specific shift values ​​are for reference only. The increase of the electrolyte will cause the Rf value of the dye to decrease, and some low-energy dyes have relatively smaller measured Rf values ​​due to relatively more Sumite powder, which does not represent high affinity. Therefore, when a dyeing plant uses multiple dyes for color matching, the selected dyes must be tested in the same system for the same affinity.

Method A selected dye is formulated into a solution at a prescribed ratio and diluted to a certain concentration (the concentration is too high to distinguish the color), and then the above method is used to measure the rise height and speed of each dye. If there is no obvious color gradation, then The dyes have similar affinities; when the rise heights are not uniform, the dye with the highest rise has the lowest affinity.

Method II filter halo method. Divide the dyestuff into a solution according to the prescription ratio, dilute to a certain concentration, and drip 1 to 2 drops (about 0.05 mL) of the dyeing solution on the medium-speed filter paper to observe the penetration and diffusion of the dyeing solution on the filter paper. The dye that penetrates and diffuses into the outer ring has a low affinity. If the color of the whole ring is uniform and there is no stratification, the affinity of the dye is very close.

Shenxinfix S-type reactive dyes have a heterobifunctional active structure. After refinement and purification, their affinity is low and they are suitable for padding, but care should be taken in selecting them.

1. Dye reaction speed

When choosing the dye, it is necessary to choose the reactivity as close as possible. If the reactivity of the selected dye differs greatly, it is not good for the color light control. If the black KNB reacts quickly, the blue KNG reaction is slower. When the speed of padding is faster, the black KNB fixation is less affected, and the blue KNG fixation rate is significantly reduced, resulting in a large change in shade. Inevitably, when the reactive dyes with poor compatibility are used, the stability of the process parameters must be strictly controlled.

1. Alkalinity of 3 dyes

Dyes containing vinyl sulfone structures have poor alkali resistance. When the fixation is too strong or the steaming time is too long, the hydrolysis of the dye increases and the color becomes lighter, such as commonly used black KNB.

If the fixing liquid is too strong or the steaming time is too long, the hydrolysis of the dye will increase and the color will become lighter. However, the alkali resistance of different dyes is also different, and the steaming process conditions are also different. For example, the black KNB is more affected than the red SBE. The influence of changes in process parameters is large. In the case of light-colored dyes, black KNB, navy and other shades are not easy to control, and they are lighter in speed. This is because the dyes have poor alkali resistance, long color fixing time, increased dye hydrolysis, and light coloration.

When the amount of dye is increased to 40 g/L or 60 g/L or more, alkalinity must be properly enhanced to increase the color depth.

2. Chromatic aberration control in normal continuous production

2. 1 equipment process capability

Equipment process capability refers to the ability of equipment to meet the needs of the production process. Some engineering and technical personnel do not pay enough attention to this. For padding and drying processes, it is considered that as long as padding is uniform, it can be dried. In fact, drying is a very important process in continuous pad dyeing. Drying the fabric slowly and uniformly helps to obtain full color and color fastness. It also helps to control the left-right color difference; Insufficient coloration of the fabric and increased dye migration prevented poor rubbing fastness and left-to-right color difference and front-to-back color difference.

In general, rolling and drying equipment consists of uniform rolling padding, far-infrared radiation pre-baking, two hot-air pre-bake, and drying drum drying. Under normal circumstances, two groups of MH662 infrared radiation pre-baking should evaporate 15% to 20% of the moisture, two groups of MH631 hot air should bake and evaporate 30% to 40% of the water, MH602 drying cylinder drying part of evaporation 30% to 40% Moisture. If a certain part of the drying does not meet the requirements, the other two parts will increase the drying load, and are prone to sudden drying, which is not conducive to controlling the color difference. When the speed or steam pressure of the padding changes, the proportion of evaporated water in each part changes, which causes the dye in the fabric to migrate to the surface, resulting in a change in the surface color of the fabric, resulting in a color difference before and after. It can be seen that changes in the drying capacity and speed of the dyeing of all parts will lead to chromatic aberration; the improper proportion of drying capacity of each part is not conducive to the control of the color difference before and after; the uneven heating of the left, right, and right of the drying equipment will produce a left middle Right color difference.

2. 2 fluctuations of process parameters

There are many reasons for the color difference. The fluctuation of the process parameters such as the vehicle speed, rolling pressure, drying pressure, temperature of each process equipment, etc. will cause the color difference before and after. Fluctuations in the height of the liquid level in the front and rear chutes also produce a color difference between the front and the back, which can be controlled by a feed level automatic control device.

At present, most factories pay attention to the change of the level of the rolling tank, but the height of the liquid level in the steam box seal and the change in the total liquid amount of the seal are often overlooked. In normal production, the dye dissolved in the sealing liquid on the fabric is re-stained onto the running fabric and the dye on the fabric is continuously dissolved into the sealing liquid. This is a dynamic equilibrium, and the balance is broken when the balance is broken. To control this dynamic balance, it is necessary to stably control the speed of the vehicle and the total volume of seals. However, keeping the level of the steam box sealing liquid highly stable does not fully guarantee that the total volume of the reaction seal is stable, because when the steam pressure of the steam box changes, it will cause the total liquid amount of the seal to change (the steam pressure becomes smaller and the total liquid volume of the seal increases. Large, as shown in the figure), broke the original dynamic balance, resulting in color aberration. The best way is to control the steam pressure stability of the steamer by controlling the stability of the water level of the U-shaped pressure display tube of the steamer. Both the steam pressure gauge and the temperature gauge are not easy to observe accurately and have errors, and the U-shaped tube intuitively reflects the pressure inside the steam box with almost no error and no temperature hysteresis.

3 Pretreatment of dyeing and control of dyeing finishing

In addition to the requirements for sizing, boiling, bleaching, and mercerizing, dyeing blanks emphasize the "uniformity". That is, the origin of the raw material, desizing effect, scouring effect, bleaching whiteness, mercerizing effect must be the same, otherwise there will be a color difference. In addition, poor control of the dye finishing process can also produce chromatic aberration.

3. 1 The cotton fiber of different origins has different maturity and different dyeing properties. The sizing components and the sizing rate of different batches of the same textile mill may not be the same. The desizing effect will be different and will naturally affect the dyeing, resulting in color difference. . Therefore, different batches of grey cloths of the same variety should also be treated separately.

3.2 Pretreatment requires desizing and scouring, and the effect is more than 8 cm/30 min. The requirements are uniform; the dyeing of the particularly bright color requires good whiteness, the dyeing of the general color requires uniform whiteness, and the mercerizing is sufficient. Uniform, especially soda clean and consistent.

3. 3 The unevenness of the pre-treatment results in the left-right color difference. The most common is the deep color in the middle and the color is light on both sides. Because different dyes have different sensitivities to pretreatment differences, the left-right color difference appears difficult to adjust even with a uniform rolling mill. Therefore, the consistency of the left, right, and right pressures of the pre-treatment rolling mill and the uniformity of the left, right, and right of the feedings in all areas cannot be ignored.

3.4 Color changes may also occur when finishing the dyed cloth, resulting in color difference before or after the left or right. Such as softeners, water repellents, anti-wrinkle agents and other processing will produce shade changes, often appear deepening or yellowing, reddish, etc.; fabric finishing needs heat setting, and many reactive dyes will produce a color change at high temperatures, Cotton fibers can also cause yellowing due to high temperatures. When the left, right, and right temperatures are uneven during shaping, there may be a left-right color difference, and unstable temperature during the operation of the setting machine can cause color aberration.

Conclusion

1. Sort the blanks for control. According to different manufacturers, batch number classification, do a good job in the laboratory analysis.

2. Strictly control the quality of the pre-treatment. The quality indexes such as desizing rate, gross effect, whiteness, pH value, mercerization value and dry humidity were controlled within a small fluctuation range (specific quantitative parameters were set according to the actual capacity of the factory).

3. Reasonably choose the reactive dye compatibility. Try to use the same type of reactive dyes, give full consideration to the dye affinity, alkali resistance, reactivity and other similar issues. Collect relevant technical data and create files.

4. Use dyeing aids correctly. Reasonable use of anti-migration agent can reduce the influence of the fluctuation of the drying process on the color light, according to the dye characteristics, choose different fixed alkali agent dosage and so on.

5. Strictly control the padding process parameters. The speed, temperature, pressure, and concentration must all be controlled within a relatively small range. The temperature of the dyeing and fixing liquids should also be relatively stable. Utilize the steamer U-tube to control the steam pressure of the steamer. Stable control of washing and soaping The amount of water and temperature, etc. (all processes require quantitative parameters and allowable error range).

6. Before the dyeing, the discoloration test must be carried out after finishing, and the control of the finishing process after dyeing is emphasized to prevent the formation of color differences after dyeing.